1. Technical Field
The present invention relates to a gel dosimeter for measuring radiation dose and manufacturing method therefor. More specifically, the present invention relates to a gel dosimeter for measuring radiation dose and manufacturing method therefor, for measuring three-dimensional dose distribution.
2. Background Art
Particle therapy using charged particle beams with high dose convergence, such as a proton beam, a heavy ion beam, including carbon and neon beams, has been introduced. The particle therapy is advantageous for its capability of controlling irradiation position and dose amount in the particle beam cancer treatment with higher accuracy than conventional X ray therapy. For the particle therapy, we should simultaneously pursue both of appropriate energy deposition from the particle beam to the target such as tumor position within the living body and suppression of damages to normal tissues surrounding the target position as much as possible. For this purpose, the spread of particle beam in diameter directions and the position of Bragg peak of the particle beam are adjusted to the target position in the irradiation object. Furthermore, a highly accurate therapy scheme called intensity modulated particle therapy (IMPT) is about to be practiced, in which accumulated doses of microscopic energy deposition at three-dimensional positions within the irradiation object, or dose distributions, are adjusted precisely. The amount of microscopic energy transfer due to ionization or excitation of molecules in a substance by a single particle is an energy deposit value per length transferred from the single particle to the substance, and is called a linear energy transfer (LET). The Bragg peak denotes a peak in the LET that is found near the endpoint of the range. Since the interaction with the substance, or the amount of ionization increases with inversely proportional to squared velocity of the incident particle, more amount of energy is deposited to the substance near the endpoint of the range by a particle such as positron or heavy ion. After accumulating the microscopic energy deposition at each point over particles, the dose amount at the point is obtained.
Dose distributions at three-dimensional points within the living tissue are optimized in treatment planning of the actual particle therapy. In typical treatment planning, a region where significant impact is exerted by the radiation is fit to an actual shape of the target tissue, on top of that the dose distribution in target tissue, i.e., point-by-point dose amount by the radiation, is also modified according to the therapy objective. At the same time, impact on surrounding healthy tissues is avoided, and impact especially on organ at risk is suppressed as much as possible. It should be noted that beams may be precisely controlled or irradiated multiple times to achieve intended treatment effect over a region with a complex shape extending in a width and a depth directions when using particle beams with high dose convergence that are usually adopted in the particle therapy. Schemes of the control may include shaping Bragg peak into the region as well as expanding a pencil beam and spreading the Bragg peak. For such purpose, particle irradiation device is equipped with auxiliary devices, like a wobbler and a ridge filter, and with filters and collimators, like a range shifter, a multi-leaf collimator, and a bolus, that will be adjusted to the irradiation object. For precise control in the radiation treatment, not only the whole device including the particle radiation device, the auxiliary devices, and the filter/collimator, but also irradiation process using such devices needs advanced quality assurance and quality control (hereinafter abbreviated to “QA/QC”).
The QA/QC regarding the treatment planning and the devices requires technology that enables measurement of actual energy depositions caused by a large number of particles incident from various directions at various acceleration energies with proper accumulation capability. This is because the three-dimensional distribution of the energy depositions, or dose distribution, will be possible and thus the QA/QC will be supported, if an accurate measurement with accumulating the energies can be carried out at each position. Conventionally, such an object has been pursued with one- or two-dimensional dosimeters, such as ionization chamber dosimeters, semiconductor detectors, or film-type dosimeters. These types of dosimeters are used to obtain dose distributions along one- or two-dimensional coordinates for a region of particle beam that should be aligned at the target position. In addition to these, gel dosimeters have attracted much attention recently, in which a three-dimensional dose distribution can be measured by a gel material that operates under a measurement principle of chemical dosimeters. It is possible with the gel dosimeters to precisely measure energy values deposited by radiation at points within water, which could be regarded as a material equivalent to a living body. In other words, the gel dosimeters are advantageous for their measurement capability of radiation effect on a living body equivalent material or a water equivalent material. As a result, gel dosimeters can be used for measurement of three-dimensional dose distribution while using it as a solid phantom.
Types of gel dosimeters known to date are classified generally into: polymer gel dosimeters, Fricke gel dosimeters, and dichromic acid gel dosimeters (dichromate gel, or DCG dosimeters). Of these types, the polymer gel dosimeters that record cross-link reaction of pre-polymer have already solved a problem caused by diffusion (see, for example, Patent Document 1). On the other hand, the Fricke gel dosimeters, which have gel of aqueous solution of ammonium iron(II) and iron(II) sulfate, and the DCG dosimeters, which have gel of aqueous solution of chromium dioxide (chromium(IV) oxide) have problems that the images therein will become indistinct over time due to diffusion after the irradiation. Thus, the inventors of the present application have tried to suppress such diffusion by mixing clay particles called nanoclay into Fricke Xylenol gel (FXG) dosimeters, a type of Fricke gel dosimeters, and into DCG dosimeters as irradiation object, and have succeeded in sustaining the recordation over time to a certain level (Non Patent Document 1).